1. Field of the Invention
The present invention relates to an apparatus and method for performing fast acquisition of PN sequences by transferring distributed samples, more particularly, to the apparatus and method for the fast acquisition of PN sequence employing a distributed sample transmission in a CDMA mobile communication system of code division multiple access techniques.
2. Background of the Related Art
In a CDMA mobile system, to provide a good quality of data communication between mobile stations, the receiver must be able to be quickly and accurately synchronize with the PN sequences transmitted from a transmitter. In the case of a cellar system, the receiver has to be synchronized to the pilot signal transmitted from the base station. More specifically, the next generation mobile communication systems should have functions providing new services, such as wireless multimedia communication for transmitting and receiving data in a group in a given frequency band by connecting between personal data terminals and mobile units, to the multiple mobile unit users. Under this communication environment, the system has to be able to effectively process a large quantity of information being simultaneously communicated between multiple users. Therefore, the spreading of data must inevitably employ a PN sequence of long period.
For example, in the CDMA mobile communication system, the transmitter unit first spreads a data stream to be transmitted over a prescribed spectrum by using a PN sequence locally generated from a shift register generator (SRG) of the unit, and then transmit the spread spectrum of data. The receiver unit receives the transmitted spread spectrum, despreads the received spread spectrum by using a PN sequence generated from the SRG of the receiver unit and then recovers the data stream. To execute this process, it is necessary for the SRG of the receiver unit to be synchronized to the SRG of the transmitter unit. The synchronization process includes two steps of which one is PN sequence acquisition (code acquisition) and the other is PN sequence tracking (code tracking). The two steps are sequentially executed.
In order to provide a fast and reliable service in the multimedia communication environment, a considerable amount of research on the fast code acquisition techniques has been exerted during the past decades.
One technique among the conventional techniques is a serial search acquisition method. The serial search acquisition method has the advantage of simple hardware, but the acquisition time is very long for a long-period PN sequence, because the acquisition time is directly proportional to the period of PN sequence. Therefore, several fast acquisition schemes have been developed at the cost of increased hardware complexity.
To improve the acquisition speed, a method keeping the basic structure of serial search system is provided, to which is added a two-step acquisition process, of which one step employs a passive matched filter to determine a temporary acquisition point and the other step employs an active correlator to verify the effectiveness of the temporary acquisition. Another method to improve the acquisition speed is a sequential test method using two threshold compare circuits in addition to the existing serial search scheme.
However, those methods based on the serial search do not have satisfactory acquisition speed since its absolute acquisition time is long when the sequence is long.
A recently developed technique based on the serial search method is disclosed in U.S. Pat. No. 5,644,591. In this technique, a large search window is offered in a first search step for a fast acquisition and a maximum value of correlation of each window is compared with a threshold value. When the maximum correlation is not larger than the threshold, the comparing process for the next window sequentially continues. When the maximum correlation of a window is larger than the threshold, that point is selected as a central point for a choice of a smaller search window and it is determined if the acquisition is true. Thus, by two windows having different size to each other, a candidate for the acquisition is first selected and then a confirmation of the acquisition is made. Therefore, a short acquisition time is expected. In spite of the short acquisition time, however, this method has a limitation in reducing the acquisition time when the period of the PN sequence is long since a serial search technique based on the correlation comparison is employed.
For a long sequence case, however, the parallel acquisition scheme may render a solution but the hardware complexity, that is, the number of active correlators or matched filters, increases to the order of code period.
To get around this problem, serial-parallel hybrid schemes have been proposed for practical use, in which a long code sequence of period N is divided into M subsequences, each of which having length N/M, and the acquisition circuit is thus composed of M parallel matched filters. The acquisition time of this hybrid scheme, however, is not improved as much as expected at the cost of increased hardware complexity.
Another method to improve the acquisition time uses a state estimation of the shift register generator. Theoretically, this method does not increase the complexity of hardware, compared with the serial search scheme, but it has a comparatively good acquisition time.
The acquisition method based on state estimation is a kind of rapid acquisition by sequential estimation that makes L consecutive hard decisions on the incoming code chips and loads them to the receiver SRG as the current SRG states. Therefore, this technique is successful in speeding up the acquisition process. On the other hand, in fact, it is not suitable for practical use because its performance rapidly deteriorates in a low SNR environment.
Furthermore, because of the unrealizable fact that a carrier phase must be obtained before the acquisition completion of PN sequence since coherent demodulation is used to determine whether the value of each sequence chip is positive or negative, the state estimation technique is not suitable for practical use.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.